Rapid mass ground transportation systems offer many benefits over non-mass transportation means, such as automobiles, particularly in metropolitan areas experiencing severe traffic congestion and pollution problems. Mass ground transportation may also be a desirable alternative for short-range as well as long-range air travel. Although there has been a general recognition of the need for a reliable, safe rapid transportation system, utilization of rapid transit systems has been hindered by the high cost of construction and operation as well as technical difficulties in developing an efficient and versatile light rail system.
Conventional approaches have not produced a light rail transportation system that is sufficiently versatile, efficient and cost-effective to be a feasible substitute for non-mass transportation and air travel alternatives. For instance, some so-called light rail systems have rather heavy transportation modules due to the use of heavy undercarriage or a heavy power system, high traction requirements, high on-board fuel requirements, or the like. Systems that rely on traction drives tend to have difficulty with steep grades. Moreover, external elements such as severe weather conditions and contaminations can pose substantial difficulty in the operation and maintenance of light rail systems. Additionally, traction drive mechanisms employing wheels tend to be very noisy and have a great deal of wear.
U.S. Pat. Nos. 6,360,670 (the '670 patent) and 7,225,743 (the '743 patent), which were issued to one of the co-inventors of the present invention and are assigned to the assignee of the present application, incorporated herein by reference, overcome some of these difficulties and disadvantages in an efficient and cost-effective light rail transportation system. In a specific embodiment disclosed in the '670 patent, a pod assembly is placed inside a guide tube, the exterior of which preferably supports and guides the vehicle as it moves along the tube. Motion is generated by providing a pressure differential inside the tube between the upstream region and the downstream region of the pod assembly. The pressure differential can be generated by a stationary power system that produces a vacuum on the downstream region or pressurizes the upstream region or both. The speed of the pod assembly is controlled by modulating the amount of gas flow through the pod, that is, from the upstream side to the downstream side of the pod. The speed of the pod assembly is increased by reducing the amount of gas flow through the pod assembly to thereby increase the thrust on it, and is decreased by permitting a larger amount of gas to flow past the pod assembly to decrease the thrust.
The '743 patent provides an improved light rail transportation system, having reduced wear and maintenance issues compared with the earlier system, that includes a propulsion unit set on rails inside the power tube. In an embodiment of the '743 patent, the propulsion unit employs a generally horizontally oriented thrust carriage that is disposed in a horizontal mid-portion of the power tube and includes horizontal, V-grooved wheels that engage and run along the interior rails of the power tube for guidance and weight support. A generally fan-shaped thrust valve defined by a multiplicity of thrust blades arranged in an umbrella-like fashion, also sometimes referred to as a “turkey valve” because of its fan-shaped configuration, is attached to the carriage of the propulsion unit and extends in the travel direction of the unit. Such fan-shaped thrust valves are much more effective in one direction than the other, thus the interior carriage generally has two such valves, one extending in each travel direction from the carriage to provide full thrust for the propulsion unit in either direction.
The free ends of the blades of the thrust valve can be radially expanded into or out of contact with the interior surface of the power tube. When extended and in engagement with the power tube wall, the extended blades form an umbrella-shaped wall (defining concave and convex wall surfaces) across the entire diameter of the power tube. As a result, when the air pressure on the concave side of this wall is greater than on the convex side, a thrust is generated that is transmitted via the thrust carriage and a magnetic coupler to the transportation vehicle on the outside of the power tube. The propulsion unit of the '743 patent employs a remotely-controlled switching mechanism to sequentially activate and deactivate the pressure and vacuum sources as the carriage moves along the power tube.
Although the transportation systems disclosed in the '670 and '743 patents are efficient and cost-effective light rail systems, there is a need to provide a more flexible system that allows variations in flow rate along the length of the power tube and to provide a system that is structurally more stable. Neither patent discloses a mechanism for providing different pressures, which control the flow rates, in separate sections or zones of the tube. In addition, the cyclical switching of the pressure and vacuum sources stresses the system and interferes with the smooth passage of the thrust valve over the connections points where the sources are joined to the power tube.